Detection, characterization and presentation of adverse airborne phenomena

ABSTRACT

Aircraft system and method detecting and present information relating to adverse airborne phenomena along an aircraft flight route. An imaging unit that includes an IR detector and a tunable spectral filter acquires IR images of the external environment, by acquiring wideband IR images when operating in a first mode and narrowband IR images respective of difference IR spectral bands when operating in a second mode. A data analysis unit detects and determines characteristics of adverse airborne phenomena in the environment based on at least the spectral signatures of environmental features in the acquired narrowband IR images. A display unit dynamically displays a visual representation of the detected adverse airborne phenomenon and its determined characteristics, overlaid onto a view of the external environment displayed to an operator of the aircraft. The visual representation may include variable visual attributes representing respective categories of characteristics of the detected adverse airborne phenomenon.

BACKGROUND OF THE INVENTION

Aircrafts at takeoff, cruise and landing are prone to adverse airbornetransparent or visible hazards. The source for these hazards is naturalphenomenon, such as volcanic eruptions, atmospheric instability and jetstreams. Most of the natural phenomena addressed here are not directlyrelated to human activity. Yet, when it comes to aircrafts cruising athigh altitudes, a wide spectrum of hazards is expected. These hazardsrange from bumpy flights, increased flight time, and higher fuelconsumption to passenger injuries and aircraft crash risks.

Volcanic eruptions are worldwide phenomenon. Some active volcanoes arewithin the global flight routes of commercial aircrafts. A volcaniceruption is characterized by a large quantity of ash particles emittedand dispersed in the atmosphere. These micro-sized particles togetherwith sand and dust can erode surfaces with which it comes into contact,and in more severe cases, can stall engines and compressors whilepenetrating into turbines and melting down rotating hot surfaces. Suchevents have led to major aircraft damages, and resulted in prolongedstoppages of aerial transportation.

penetration of micro-particles into the aircraft cabin may risk thephysical wellbeing of the aircraft passengers by adversely impacting therespiratory system, initiating lung diseases, or other healthcomplications. A trail of volcanic ash is often accompanied with othermore volatile materials, such as: acids and halogens. Sulfur dioxide(SO₂) and sulfuric acid (H₂SO₄) are two examples of eruption by-productsthat pose high risk to the aircrafts and to passengers. Sulfuric acid(H₂SO₄) is a highly corrosive acid, which can gradually damage variousaircraft components. In contrast, halogens exposed to UV sunlight canform radicals, which harm the ozone as well as other materials.

Although volcanic ash clouds are frequently monitored by aerialinstruments, there is a lack of real time, en-route, altitude specificinformation, which is needed for rapid assessment and maneuvering.

The presence of water ice particles is common at cruising flightaltitudes. At cruising altitudes, water can exist as an undercooledliquid without freezing. When an aircraft impacts a cloud of undercooledwater, the undercooled water will undergo a sudden freezing and forminto ice. As a consequence, ice will accumulate on the aircraft body,which elevates both the weight and air drag of the aircraft.

Aircrafts may experience turbulent shear forces when encounteringvertical air currents of an approximate scale of 100 m to 2 km. One ofthe physical mechanisms for such shear forces stems from convectivelyinduced turbulences around clouds and thunderstorms. These turbulencesare visible to the naked eye and to onboard radars, and thus may beeasily avoided. Another source for instability occurs in clearatmospheric conditions and is referred to as “clear air turbulence(CAT)”. It is assumed that man-made global warming partially contributesto these shear instabilities, which are expected to become more frequentin the impending future. Aircrafts encountering these invisible verticalshear forces may undergo sudden unpredictable movements andaccelerations, resulting in the sensation of a “bumpy” flight, as wellas potential injuries to aircraft crew members and passengers, and inextremes cases even fatalities. According to some estimates, aircraftsspend 3% of their cruising time in light intensity CATs and 1% of theircruising time in moderate CATs. Beyond their effect on passengers, CATsmay also elicit structural damage of the aircraft and increase fuelconsumption. To date, clear air turbulences are generally undetectableby onboard radars or satellites.

A current technology for turbulence detection is the enhanced X-bandradar, with two main competitive systems: the Rockwell Collins WXR-700series, and the Honeywell RDR-4 A/B series. These systems have two modesfor detection of both wind shear and convective turbulence. However,based on market assessment of forward looking turbulence sensingsystems, these systems were reported not to withstand expectations.

In recent years, Doppler LIDAR systems using phase information todetermine vertical flow velocity of air lamella are under development,yet no commercial products are available. These systems rely on laserradiance emitted along the direction of flight route, monitoring thebackscattering reflections of atmospheric molecules, such as carbondioxide (CO₂) and oxygen (O₂). Two of the main disadvantages of LIDARsystems for CAT detection are narrow FOV, and limited laser power,resulting in limited detection range and angle. These drawbacks make itdifficult for the pilot to obtain a complete image of the phenomenon andits extent. Lasers that operate at approximately 4-20 millijoules (mJ)allow for a detection range of about 5 miles, which is insufficient forrisk assessment and decision making. Increasing the laser power,however, would lead to surplus weight as well as eye safety issues.

U.S. Pat. No. 7,109,912 to Paramore et al, entitled: “Weather radarhazard detection system and method”, discloses an aircraft weather radarsystem that includes a radar antenna, optical aircraft sensors, adatabase, a processing device, and a cockpit display. The processingdevice receives radar returns from the radar antenna and environmentalvariables from the aircraft sensors, and detects storm system hazardsusing a cell height parameter for a cell. The cell height parameter isdetermined by determining a direction to the cell using theenvironmental variables, and a range to the cell using the radarreturns. The storm system hazards are displayed using an iconal ortextual representation. The hazards may include: overshooting tops,vertical development, hail, vaulted thunderstorms, air mass stability,or cell growth rate.

U.S. Pat. No. 5,357,263 to Fischer et al, entitled: “Display instrumentfor aircraft for showing the aircraft orientation, particularly therolling and pitching position or the flight path angle”, is directed toa head-up display of an aircraft which displays symbol featuresrepresentative of the aircraft position in the exterior field of view ofthe pilot. The symbol features include a symbol stabilized in anearth-fixed manner for showing the true horizon position, as well assymbols representing the rolling angle, pitching angle, or flight pathangle of the aircraft. A display of the attitude is generated by meansof a reference symbol representing the aircraft and an informationsymbol which changes with respect to the reference symbol in itsposition and shape, and is dependent on the rolling angle and pitchingangle or path angle.

U.S. Patent Application No. 2010/0231705 to Yahav et al, entitled:“Aircraft landing assistance”, discloses an enhanced vision system forassisting aircraft piloting. An aircraft control operator sends flightinstructions associated with an object of interest to a pilot wearing ahead-mounted display (HMD). A visual representation of the flightinstructions with the object of interest marked is generated, respectiveof a combined spatial and symbolic image viewed by the pilot on the HMD.The aircraft control operator receives from the pilot confirmation ofthe flight instructions by designating the marked object of interest onthe combined spatial and symbolic image, where the designation isperformed in conjunction with the line-of-sight of the pilot.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there is thusprovided an aircraft system for detecting and presenting informationrelating to adverse airborne phenomena along a flight route of theaircraft. The system includes a data acquisition unit, a data analysisunit, and a display unit. The data acquisition unit includes an imagingunit configured to acquire a plurality of infrared (IR) images of theexternal environment of the aircraft. The imaging unit includes an IRdetector and a tunable spectral filter (TSF). The IR detector isconfigured to detect IR radiation. The TSF is configured to selectivelytransmit at least one IR wavelength band of incident light to the IRdetector. The imaging unit is configured to acquire wideband IR imagesof the environment when operating in a first mode, and configured toacquire narrowband IR images of the environment, each of the narrowbandIR images respective of a different IR spectral band, when operating ina second mode. The data analysis unit is configured to receive andprocess the IR images acquired by the imaging unit, to detect anddetermine at least one characteristic of at least one adverse airbornephenomenon in the environment, based on at least the spectral signaturesof environmental features in the narrowband IR images. The display unitis configured to dynamically display a visual representation of thedetected adverse airborne phenomenon and determined characteristicthereof, overlaid onto a view of the external environment displayed toan operator of the aircraft. The data acquisition unit may obtaininformation relating to the external environment from at least one of: aweather/climate forecast model; a 3D geographic model; a digital terrainelevation model (DTEM); a ground/satellite observation station; a birdmigration information source; and/or another airborne platform. Thedetected adverse airborne phenomenon characteristic may include: aclassification type, a location, a motion trajectory, a severity leveland/or a technique for avoiding or mitigating the effects of thedetected adverse airborne phenomenon. The data analysis unit may furthercompare the spectral signatures with predefined values or thresholds, todetermine characteristics of the detected adverse airborne phenomenon.The visual representation may include a plurality of variable visualattributes, each of the visual attributes representing a respectivecategory of the characteristics of the detected adverse airbornephenomenon. The visual attributes may include: at least one symbol; atleast one color; at least one color attribute; at least one contour; atleast one shape; at least one alphanumeric character; a textnotification; a highlighting indication; a flashing visual attribute;and/or a varying visual attribute. The display unit may include ahead-up display (HUD), a head-mounted display (HMD), and/or a displayembedded within a wearable apparatus. The first mode may include ascanning mode, in which the TSF is tuned to transmit all detectable IRwavelengths, and the IR detector is configured to acquire multispectralwaveband images, where the data analysis unit is directed to detect allpotential adverse airborne phenomena in the environment. The second modemay include an investigation mode, in which the TSF is tuned to transmitselected IR wavelength bands, and the IR detector is configured toacquire narrowband images, where the data analysis unit is directed todetermine further information relating to a potential adverse airbornephenomenon detected when operating in the first mode. The imaging unitmay be further configured to operate in a calibration mode, in which theTSF is tuned to block radiation from both directions, and the TSF ispositioned so as to reflect radiation emitted from a black-body surfaceat preselected temperatures toward the IR detector, allowing for offsetand gain calibrations of the TSF. The data analysis unit may generate acontrast image from a plurality of narrowband images at differentwavelength bands acquired by the imaging unit, and to determine aspectral signature from the contrast image. The adverse airbornephenomenon may include: a selected quantity of atmospheric particles; aselected quantity of volcanic ash particles; a selected quantity of acidor halogen molecules; a selected quantity of ice particles; a selectedquantity of water droplets; a selected quantity of undercooled waterdroplets; a turbulence airflow; a vertical shear force; a convectivelyinduced turbulence condition; a clear air turbulence (CAT) condition;and/or the presence of birds.

In accordance with another aspect of the present invention, there isthus provided a method for detecting and presenting information relatingto adverse airborne phenomena along a flight route of an aircraft. Themethod includes the procedure of acquiring a plurality of infrared (IR)images of the external environment of the aircraft, using an imagingunit that includes: an IR detector, configured to detect IR radiation;and a tunable spectral filter (TSF), configured to selectively transmitat least one IR wavelength band of incident light to the IR detector, byacquiring wideband images of the environment when operating in a firstmode, and acquiring narrowband IR images of the environment, each of thenarrowband IR images respective of a different IR spectral band, whenoperating in a second mode. The method further includes the procedure ofprocessing with a data analysis unit the IR images acquired by theimaging unit, to detect and determine at least one characteristic of atleast one adverse airborne phenomenon in the environment, based on atleast the spectral signatures of environmental features in thenarrowband IR images. The method further includes the procedure ofdynamically displaying a visual representation of the detected adverseairborne phenomenon and determined characteristic thereof, overlaid ontoa view of the external environment displayed to an operator of theaircraft. The detected adverse airborne phenomenon characteristic mayinclude: a classification type, a location, a motion trajectory, aseverity level and/or a technique for avoiding or mitigating the effectsof the detected adverse airborne phenomenon. The visual representationmay include a plurality of variable visual attributes, each of thevisual attributes representing a respective category of thecharacteristics of the detected adverse airborne phenomenon. The visualattributes may include: at least one symbol; at least one color; atleast one color attribute; at least one contour; at least one shape; atleast one alphanumeric character; a text notification; a highlightingindication; a flashing visual attribute; and/or a varying visualattribute. The first mode may include a scanning mode, in which the TSFis tuned to transmit all detectable IR wavelengths, and the IR detectoris configured to acquire multispectral waveband images, where the dataanalysis unit is directed to detect all potential adverse airbornephenomena in the environment. The second mode may include aninvestigation mode, in which the TSF is tuned to transmit selected IRwavelength bands, and the IR detector is configured to acquirenarrowband images, where the data analysis unit is directed to determinefurther information relating to a potential adverse airborne phenomenondetected when operating in the first mode. The method may furtherinclude the procedure of performing offset and gain calibrations of theTSF, by operating the imaging unit in a calibration mode, in which theTSF is tuned to block radiation from both directions, and the TSF ispositioned so as to reflect radiation emitted from a black-body surfaceat preselected temperatures toward the IR detector. The IR imageprocessing may include generating a contrast image from a plurality ofnarrowband images at different wavelength bands acquired by the imagingunit, where a spectral signature is determined from the contrast image.The adverse airborne phenomenon may include: a selected quantity ofatmospheric particles; a selected quantity of volcanic ash particles; aselected quantity of acid or halogen molecules; a selected quantity ofice particles; a selected quantity of water droplets; a selectedquantity of undercooled water droplets; a turbulence airflow; a verticalshear force; a convectively induced turbulence condition; a clear airturbulence (CAT) condition; and/or the presence of birds.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with thedrawings in which:

FIG. 1 is a schematic illustration of an aircraft system for detecting,characterizing, and presenting information relating to adverse airbornephenomena, constructed and operative in accordance with an embodiment ofthe present invention;

FIG. 2 is a schematic illustration of the imaging unit of the dataacquisition unit of the system of FIG. 1, constructed and operative inaccordance with an embodiment of the present invention;

FIG. 3 is an illustration of the tunable spectral filter of the imagingunit of FIG. 2, constructed and operative in accordance with anembodiment of the present invention;

FIG. 4 is an illustration of the tunable spectral filter of FIG. 3adjusting the inter-mirror separation distance to change the transmittedradiation wavelength, operative in accordance with an embodiment of thepresent invention;

FIG. 5 is a graph showing narrow passbands as a function of appliedvoltage;

FIG. 6 is a schematic illustration of the image unit of FIG. 2performing offset and gain calibration using a black body reference,operative in accordance with an embodiment of the present invention;

FIG. 7A is a graph of transmission as a function of wavelength for atunable spectral filter operating at a closed mode, in accordance withan embodiment of the present invention;

FIG. 7B is a graph of transmission as a function of wavelength for atunable spectral filter operating at a high dynamic range mode, inaccordance with an embodiment of the present invention;

FIG. 7C is a graph of transmission as a function of wavelength for atunable spectral filter operating at a high resolution mode, inaccordance with an embodiment of the present invention;

FIG. 8 is a flow diagram of a method for detecting, characterizing, andpresenting information relating to adverse airborne phenomena, operativein accordance with an embodiment of the present invention;

FIG. 9 is an illustration of an exemplary spectral signature extractedfrom multiple narrowband images acquired at different spectral bands,operative in accordance with an embodiment of the present invention;

FIG. 10 is an illustration of an exemplary spectral response extractedfrom a narrowband image changing over time, operative in accordance withan embodiment of the present invention; and

FIG. 11 is a schematic illustration of a dynamic display of detectedadverse airborne phenomena, constructed and operative in accordance withan embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention overcomes the disadvantages of the prior art byproviding a system and method for detecting and presenting informationrelating to adverse airborne phenomena along a flight route of anaircraft. The system includes data acquisition which collects data fromvarious external and onboard data sources, including an infrared imagingunit with a Fabry-Pérot based tunable spectral filter that acquireswideband and narrowband IR images of the environment. The system furtherincludes data analysis which involves a spectral analysis of thenarrowband images at different wavelength bands for identifying adverseairborne phenomena. The system further includes dynamic display of arepresentation of the detected adverse airborne phenomena, overlaid overa scene view of the external environment, with variable symbols andother supplementary visual attributes depicted to indicate variouscharacteristics of the detected phenomena. The adverse airbornephenomena may include, for example: volcanic ash clouds; dust; sand;airborne molecules and aerosols; other atmospheric particles; icecrystals or ice coated particles; super-cooled water particles; waterdroplets; uncooled water droplets; clear air turbulence (CAT); a CATenvironmental condition; a turbulence airflow; a vertical shear force; aconvectively induced turbulence environmental condition; wake vortices;and/or the presence of birds.

Reference is now made to FIG. 1, which is a schematic illustration of anaircraft system, generally referenced 100, for detecting, characterizingand presenting information relating to adverse airborne phenomena,constructed and operative in accordance with an embodiment of thepresent invention. System 100 includes three major sub-systems: dataacquisition unit 110, data analysis unit 120, and display unit 130. Dataacquisition unit 110 includes an imaging unit 115 that captures imagesof the external environment. Data acquisition unit 110 may also acquireadditional data simultaneously from various external and on-boardsources. The acquired data is transferred to analysis unit 120 in rawform or after undergoing initial processing. Data analysis unit 120analyzes and evaluates the collection of data and the acquired images toidentify different adverse airborne phenomena in the vicinity of theaircraft. Data analysis unit 120 may receive a digital representation ofthe images acquired by imaging unit 115, or may convert the raw imagedata into brightness temperature images. The image analysis may be basedon a single image or multiple images. Data analysis unit 120 processesthe data to determine further information, such as the severity levelposed by the phenomenon to the aircraft and the aircraft passengers, thelocation of the phenomenon with respect to the aircraft, and thedirection and velocity of the phenomenon. Data analysis unit 120 mayalso determine a predicted trajectory of the adverse airborne phenomenonwith respect to the aircraft. Assessment of such characteristics may bebased on thresholds, dynamic thresholds and/or lookup tables. To thisend, data analysis unit 120 employs suitable algorithms for dataextraction and data matching. Data analysis unit 120 also generates animage that provides a visual representation of the detected phenomenaalong with their associated characteristics.

Display unit 130 receives the analyzed data from data analysis unit 120and provides a visual representation to the aircraft pilot (and/or otheraircraft crew members), manifesting the most important characteristicsof the detected adverse airborne phenomena. The characteristics may bedisplayed using symbology and by employing different visual attributes,such as different colors, shapes, contours, audible cues, textnotifications, animations, and the like. Information of particularimportance may be emphasized to the pilot, such as the concentration andestimated severity level of the phenomenon, the wind direction, and apredicted motion trajectory of the phenomenon. The characteristics maybe transmitted to and stored at a ground station database. The storeddata may be used for classification and evaluation of the detectedadverse airborne phenomena, as well as generating a global mapping ofthese phenomena. The generating maps may then the be consulted fordetermining safe flight routes, helping to decrease aircraft fuelconsumption and reduce the likelihood of encountering adverse airbornephenomena over the course of a flight.

Data acquisition unit 110 may collect on-line data from both externaland on-board sources. For example, the collected data may include:weather or climate forecasts, bird migration data, aircraft flightroutes, a three-dimensional (3D) geographic model or virtual globe, anddigital terrain elevation data (DTED). The external data may beobtained, for example: from another airborne platform; from a ground orsatellite observation station; from a geographic model or a digitalterrain elevation map; from a volcanic ash advisory center (VAAC); froma publically available news source; and the like. The collected data isthen processed by data analysis unit 120 to allow for the detection andcharacterization of adverse airborne phenomena, which may then be visualrepresented by display unit 130.

Data acquisition unit 110 further includes an electro-optical basedimaging unit 115. Reference is now made to FIG. 2, which is a schematicillustration of the imaging device 115 of the data acquisition unit 110of the system 100 of FIG. 1, constructed and operative in accordancewith an embodiment of the present invention. Imaging unit 115 includesan optical shutter 116, a tunable spectral filter (TSF) 117, and aninfrared (IR) detector 118. TSF 117 and IR detector 118 are positionedbehind optical shutter 116 along a common optical path. TSF 117 may beremoved from the optical path if imaging unit 115 is intended to acquirefull wideband images (in the IR spectral range). In order to acquirenarrowband spectral images, TSF 117 is introduced into the optical pathand tuned to the desired band. The precise location of TSF 117 along theoptical path is design dependent, although usually positioned in frontof a collimated beam.

IR detector 118 may be embodied by an uncooled micro-bolometer sensorwith an active material such as vanadium-oxide (VOx). The uncooledsensor may be sensitive to infrared radiation at the long-wavelength IRregion (e.g., between approximately 7 μm-14 μm). It is appreciated thatdetection in the infrared range is described herein for exemplarypurposes, and that imaging unit 115 may generally include at least onesensor configured to acquire any form of electromagnetic radiation atany range of wavelengths (e.g., light in the visible or non-visiblespectrum, ultraviolet, infrared, radar, microwave, RF, and the like).

Reference is now made to FIG. 3, which is an illustration of the TSF 117of the imaging unit 115 of FIG. 2, constructed and operative inaccordance with an embodiment of the present invention. TSF 117 is basedon a Fabry-Pérot interferometer, which includes two semi-transparentmirrors 141, 142, spring elements 143, 144, and actuators 146, 147. Eachmirror 141, 142 includes a partially-reflective surface, which arealigned in parallel and separated by an optical gap defining aFabry-Pérot cavity (referenced 145) in between. The optical gap may becomposed of air or another medium through which light may pass through.

Incident light is reflected within the cavity 145 between mirrors 141and 142, and light of a particular wavelength is transmitted from TSF117, in accordance with the principles of a Fabry-Pérot interferometer.In particular, a broad spectrum input light beam (referenced 148) thatincludes a plurality of wavelengths (λ₁ . . . λ_(n)) undergoes multiplereflections within cavity 145 between mirrors 141 and 142. The reflectedbeams interfere with each other, including constructive interferenceoccurring when the beams are in phase, and destructive interference whenthe beams are out of phase. Whether the beams are in phase is a functionof the inter-mirror or optical gap width, i.e., the separation distancebetween the two mirrors, as well as the light wavelength, the angle atwhich the light is incident on the mirror, and the refractive index ofthe separation medium. As a result, the light beam (referenced 149)exiting TSF 117 is a narrowband light beam having a selected wavelength(λ_(i)) that is a function of the width of optical gap 145.

Actuators 146, 147 and spring elements 143, 144 are operative to adjustthe width of optical gap 145 in order to provide the desired outgoingwavelength. Spring elements 143, 144 are positioned between the parallelmirror surfaces along the outer periphery, and provide acounterbalancing force opposing forces acting to decrease the opticalgap 145. Spring elements 143, 144 may be embodied by a thin strip ofsilicon, or another material that provides flexibility and support.Actuators 146, 147 are also disposed along the periphery of mirrors 112,142 between the parallel surfaces, and are configured to apply a uniformforce for moving one mirror relative to the other. In particular, eachactuator 146, 147 applies an electromagnetic force which pushes or pullsmirror 142 (toward or away from mirror 141), while the respective springelement 143, 144 provides a resilient opposing mechanical force, therebymaintaining the spacing (optical gap width) between mirrors 141 and 142.The equilibrium between the electromagnetic force from actuators 146,147 and the mechanical force from spring elements 143, 144 determinesthe location of mirrors 141 and 142, and correspondingly, the opticalgap width, which defines the wavelength of exiting light beam 149.

Common types of actuators include electrostatic actuators (e.g.,capacitors), piezoelectric actuators, and magnetic actuators.Considering for example electrostatic actuation, the optical gap ismaximal when there is no actuation force, i.e., at zero voltage. Byapplying voltage between the actuators, the inter-mirror optical gapwill tend to decrease. For electrostatic actuation, the applied forcefor decreasing the optical gap between the mirrors may be formulated as:

$\begin{matrix}{{F_{electrostatic} = \frac{ɛ_{0}{AV}^{2}}{2\; d^{2}}};} & \left( {{Eqn}.\mspace{14mu} 1} \right)\end{matrix}$

where: ε₀ represents the vacuum permittivity constant; A represents thearea of the actuator (actuation pad); V represents the applied voltage;and d represents the optical gap width.

Reference is made to FIG. 4, which is an illustration of the tunablespectral filter 117 of FIG. 3 adjusting the inter-mirror separationdistance to change the transmitted radiation wavelength, operative inaccordance with an embodiment of the present invention. FIG. 4 depicts achange in the inter-mirror gap 145 as a result of applied actuation,where the gap width has decreased by the amount “x”. As a result, thepassing band of the emitted radiation 149 (i.e., the range of wavelengthpassing through TSF 117) will also be changed accordingly. Reference ismade to FIG. 5, which is a graph showing narrow passbands as a functionof applied voltage.

As with other optical elements, tunable filters may be subject tochanges in ambient conditions such as temperature and humidity drifts.The changing ambient conditions can affect the response of the materialscomposing the tunable filter, and thereby influence the optical gapwidth 145 and the passband accuracy. Accordingly, TSF 117 may becalibrated at different ambient conditions in order to correct thefluctuations of the optical gap width resulting from different ambientconditions (discussed further below with reference to FIG. 6).Alternatively, imaging unit 115 may include a real-time correctioncircuit, configured to continuously obtain data relating to the positionof mirrors 141, 142, based on which the inter-mirror gap can be adjustedas necessary. For example, electrodes deposited on the surfaces of eachof mirrors 141, 142 may continuously measure the capacitance between themirror surfaces, from which the inter-mirror gap width can be deduced,allowing for accurate monitoring and adjustment of the wavelength bandof transmitted radiation 149.

The transmitted radiation through a Fabry-Perot based tunable spectralfilter is given by the following equation:

$\begin{matrix}{{T = \frac{1}{1 + {{F \cdot \sin^{2}}\theta}}};} & \left( {{Eqn}.\mspace{14mu} 2} \right)\end{matrix}$

where: F represents the contrast factor defined by the reflectance R ofthe semi-transparent mirrors, as follows:

$\begin{matrix}{{F = \frac{4\; R}{\left( {1 - R} \right)^{2}}};} & \left( {{Eqn}.\mspace{14mu} 3} \right)\end{matrix}$

and where the phase (θ) for normal incidence of radiation is defined by:the refractive index of the optical gap (n); the optical gap width (d);and the wavelength (λ), as follows:

$\begin{matrix}{\theta = \frac{2\pi\;{nd}}{\lambda}} & \left( {{Eqn}.\mspace{14mu} 4} \right)\end{matrix}$

For full transmittance, the sin²θ factor will vanish, i.e., constructivestanding waves are formed between the two semi-transparent mirrors ofthe filter. This is achieved when θ=πm, where m represents an integerdefining the order of the transmitting peaks.

TSF 117 is capable of operating in different operational modes. A firstoperational mode can be considered a “closed mode” or a “calibrationmode”. At a certain inter-mirror gap width, TSF 117 will block radiationat both directions, such that no radiation within the defined spectralrange (e.g., LWIR) will pass through TSF 117 (i.e., any wavelength inthe defined spectral range will be reflected). At this state it ispossible to calibrate the offset and gain of IR detector 118. Referenceis made to FIG. 6, which is a schematic illustration of the image unit115 of FIG. 2 performing offset and gain calibration using a black-bodyreference surface, operative in accordance with an embodiment of thepresent invention. TSF 117 is inclined along an optical axis between thescene radiation and IR detector 118, and reflects radiation emitted froma black-body surface 152 at pre-selected temperatures, toward IRdetector 118. In this manner, offset and gain calibrations can beperformed as a function of a single or multiple black-body temperatures,respectively. Typically, such calibration is performed once every fewminutes. The calibration may be performed for offset only, or for bothoffset and gain. The effect of self-radiation of TSF 117 may beminimized by thermally coupling TSF 117 through a heat-pipe (or anotherpassive or active cooling mechanism) to the exterior ambient. Thisthermal coupling may be particularly effective for decreasingself-radiation at cruising altitudes where the temperatures are belowzero Celsius degrees. Furthermore, in order to calibrate IR detector 118to the temperatures in the imaged scene, black-body surface 152 may bethermally coupled to the exterior ambient using a heat pipe (or anotherpassive or active cooling mechanism). Reference is made to FIG. 7A,which is a graph of transmission as a function of wavelength for atunable spectral filter operating at a closed mode, in accordance withan embodiment of the present invention.

Another operational mode of TSF 117 is a “high sensitivity mode” or“high dynamic range mode”, in which multiple wavelengths in the definedspectral range (e.g., LWIR) are transmitted through TSF 117. In thismode, the optical gap width of TSF 117 is set so as to allow theformation of multiple transmitting peak orders (m=1, 2, . . . ),resulting in IR detector 118 acquiring images composed of multiplespatially separated wavebands. The simultaneous acquisition ofmulti-spectral waveband images (“wideband images”) serves to increasethe likelihood of an initial detection of an adverse airborne phenomenon(or lowering the detection time) since image data is acquired in a widedynamic range, but at the expense of lowered spatial resolution (i.e.,decreased precision in determining the location of the adverse airbornephenomenon). Reference is made to FIG. 7B, which is a graph oftransmission as a function of wavelength for a tunable spectral filteroperating at a high dynamic range mode, in accordance with an embodimentof the present invention.

A further operational mode of TSF 117 is a “high resolution mode”, inwhich TSF 117 passes through a single wavelength band in the definedspectral range (e.g., LWIR), such that IR detector 118 captures a singlespectral waveband image (“narrowband image”) at a time. This mode allowsfor determining the location of an adverse airborne phenomenon in theimaged scene with a higher degree of precision (i.e., higher spatialresolution), but at the expense of not detecting other adverse airbornephenomena having a characteristic spectral response in a differentspectral band. Reference is made to FIG. 7C, which is a graph oftransmission as a function of wavelength for a tunable spectral filteroperating at a high resolution mode, in accordance with an embodiment ofthe present invention.

The “high dynamic range” mode and the “high resolution” mode aregenerally complementary, and TSF 117 may generally repeatedly switchbetween these modes in an iterative manner to enhance the overallprocess of detecting and characterizing multiple adverse airbornephenomena. In particular, the high dynamic range mode of TSF 117 mayalso be considered as a “scanning mode”, in which imaging unit 115 isdirected to detect all potential adverse airborne phenomena by acquiringwideband images in all available (IR) wavelength bands. Correspondingly,the high resolution mode of TSF 117 can be considered an “investigationmode” in which imaging unit 115 is directed to acquire only narrowbandimages in selected (IR) wavelength bands (i.e., one or more discretespectral bands), in order to determine further information and moreaccurate information relating to a potential adverse airborne phenomenonpreviously detected in the scanning mode.

For example, if data acquisition unit 110 receives minimal preliminaryinformation about potential adverse airborne phenomenon to beencountered along the flight route of the aircraft, then imaging unit115 may operate initially in a “scanning mode” (or high dynamic rangemode) so as to scan the scene for various adverse airborne phenomenawith equal probability. When the presence of a specific adverse airbornephenomenon is detected, such as by imaging unit 115 operating inscanning mode and/or by information obtained from an external datasource, then imaging unit 115 may be directed to operate in an“investigation mode” (or high resolution mode) in order to concentrateon collecting further information on the detected phenomenon (such asits spatial location). For example, imaging unit 115 may tentativelydetect a potential adverse airborne phenomenon when operating in ascanning operational mode, and then positively verify (and obtainfurther characteristics of) the tentatively detected adverse airbornephenomenon when operating in an investigation operational mode.Investigation mode operation allows for enhanced data collection andanalysis, in order to decrease the likelihood of false alarms (i.e.,falsely detecting an adverse airborne phenomenon when one is notactually present). Imaging unit 115 may subsequently go back tooperating in scanning mode, followed by another investigation mode afterdetecting a new adverse airborne phenomenon, repeating iteratively asnecessary. In this manner, the detectability level (i.e., the quality ofthe detections, such as the amount and the accuracy of informationobtained) is increased, and the detection time (or number of “misseddetections”) is decreased. Increasing detectability may also be achievedby scanning selected spectral bands (of relevant potential adverseairborne phenomena) more frequently, thereby effectively increasing thesignal to noise ratio (SNR), and/or by focusing on a selected region ofinterest (ROI) in the imaged scene.

Reference is now made to FIG. 8, which is a flow diagram of a method fordetecting, characterizing, and presenting information relating toadverse airborne phenomena, operative in accordance with an embodimentof the present invention. In procedure 162, information relating to theexternal environment along an aircraft flight route is obtained fromexternal or onboard data sources. Referring to FIG. 1, data acquisitionunit 110 obtains data relating to the environment in the vicinity ofsystem 100, such as weather or climate forecasts, bird migration data,aircraft flight routes, a 3D geographic model or virtual globe, and/ordigital terrain elevation data.

In procedure 164, infrared images of the environment along the aircraftflight route is acquired with an imaging unit that includes aFabry-Pérot based TSF. Referring to FIGS. 1 and 2, data acquisition unit110 includes an imaging unit 115 that includes TSF 117 and IR detector118. Imaging unit 115 acquired wideband and narrowband infrared imagesof the environment in the vicinity of system 100. In particular, insub-procedure 166, imaging unit 115 operates in a scanning mode toacquire wideband images. When imaging unit 115 operates in a scanningmode or high dynamic range mode, the optical gap width of TSF 117 is setto transmit multiple IR wavelengths such that IR detector 118 capturesmultiple images of spatially separated (IR) wavelength bands. If forexample it is desired to focus on a specific adverse airborne phenomenondetected in a scanning mode so as to obtain the spatial location orother information, as well as to improve the accuracy of the obtainedinformation, imaging unit 115 may be directed to operate in aninvestigation mode or a high resolution mode. Accordingly, TSF 117 istuned to one or more selected discrete spectral bands (sub-procedure168) associated with the specific adverse airborne phenomenon. As aresult, IR detector 118 captures narrowband (IR) images in the selectedwavelength bands, allowing imaging unit 115 operating in the highresolution mode (sub-procedure 170) to determine further characteristicsof the specific adverse airborne phenomenon, such as its location.Subsequently, TSF 117 may be re-tuned to pass through multiplewavelengths to enable wideband imaging (sub-procedure 172), and imagingunit 115 returns to scanning mode operation for detecting new adverseairborne phenomena. Accordingly, imaging unit 115 iteratively switchesbetween scanning mode operation and investigation mode operation asrequired, to enhance the overall detection and characterizationcapabilities. Imaging unit 115 may also undergo recurrent calibration,such as using a black-body surface 152 set at pre-selected temperatures(FIG. 6), to allow for offset and gain calibrations.

The images acquired by imaging unit 115 are transferred to data analysisunit 120 for processing and detection and characterization of adverseairborne phenomena. The sequence of filter tuning, image acquisition,and image transfer to data analysis unit 120 is repeated continuouslyfor each of the selected narrowbands in which imaging unit 115 operates.The number of acquired images depends on various parameters, such as thesensitivity of IR detector 118, the concentration of the airbornephenomenon to be detected, and the like.

In procedure 174, the collected data and the acquired images areprocessed to detect and determine characteristics of at least oneadverse airborne phenomenon, based on at least the spectral response atdifferent wavelength bands of features in the narrowband images.Referring to FIG. 1, data analysis unit 120, which is synchronized withIR detector 118 and TSF 117, receives the acquired IR images and othercollected data from data acquisition unit 110. The images are stored forfurther processing and data extraction. In particular, data analysisunit identifies the spectral response of different regions orenvironmental features present in the narrowband images. The uniquespectral response at different wavelength bands is known as a “spectralsignature”. Each airborne phenomenon is characterized by a uniquespectral signature. Accordingly, imaging unit 115 generally acquiresmultiple narrowband images at different spectral bands, so as to enablethe determination of a characteristic spectral signature and to decreasethe false alarm rate (FAR).

The spectral signature of the airborne phenomena depends on the natureof the interaction of the LWIR photons with the particles or physicalelements that make up the particular phenomenon. Reference is made toFIG. 9, which is an illustration of an exemplary spectral signatureextracted from multiple narrowband images acquired at different spectralbands, operative in accordance with an embodiment of the presentinvention. Image 182 is acquired at a first spectral band “X”, image 184is acquires at a second spectral band “Y”, and image 186 is acquired ata third spectral band “Z”. Each image 182, 184, 186 is characterized bya particular spectral response at a different wavelength band. Whenthese spectral responses at different spectral bands are processedtogether, a unique spectral signature is obtained. This spectralsignature forms the basis for the detection and identification ofadverse airborne phenomena.

For particles at the molecular level, the type of interaction (betweenthe LWIR photons and the environmental particles) is usually absorption.For example, considering silicon dioxide (SO₂) and ozone (O₃), there aretwo distinct absorption bands associated with SO₂ that appear in theLWIR range: at approximately 7.3 μm and 8.6 μm. Accordingly, dataanalysis unit 120 may examine the spectral response at each of thesebands (or both) to identify the characteristic spectral response ofsilicon dioxide.

In the case of micro-sized particles, the type of interaction is usuallya combination of absorption, scattering and transmission. For example,volcanic ash clouds of aged ash particles may have a characteristicparticle diameter of approximately 3-10 μm. For detecting such ashparticles, a contrast image may be generated from two narrowband imagesacquired at two different wavelengths, e.g., 11 μm and 12 μm. Volcanicash particles have a distinct wavelength-dependent refractive index(usually opposite to that of water), which may be easily apparent in thebackground of the processed contrast image.

Another physical mechanism which can be utilized for detecting certainphenomena is the reststrahlen effect, in which radiation within a narrowwavelength band cannot propagate within a particular medium due to achange in refractive index concurrent with the specific absorption bandof the medium. As a result, a reststrahlen band radiation will undergostrong reflection from that medium. The reststrahlen effect isresponsible for a unique spectral signature at a wavelength of 8.2 μmwhich is associated with quartz. Therefore, monitoring this particularwavelength can provide an indication of the presence and concentrationof quartz, which is one of the volcanic ash constituents.

The identified spectral signature may also take into account changes inthe spectral response over time. Reference is made to FIG. 10, which isan illustration of an exemplary spectral response extracted from anarrowband image changing over time, operative in accordance with anembodiment of the present invention. Image 187 is acquired (at aparticular spectral band) at a first time instance “t₁”, while image 188represents image 187 acquired at a later time instance “t₂”=t₁+Δt. Image187 depicts a particular spectral response of an environmental featurepresent in the captured image, while image 188 depicts the same spectralresponse shifting location, for example indicating a shift in thelocation of the particular feature between time “t₁” and time “t₂”.

Data analysis unit 120 may compare the multi-source processed data withknown values and thresholds in order to obtain various characteristicsassociated with a detected adverse airborne phenomenon, such as anestimate of severity level (e.g., the level of danger posed to theaircraft and/or the aircraft passengers by the phenomenon). Dataanalysis unit 120 may also determine relevant characteristics from theidentified spectral signature of the phenomenon. The determinedcharacteristics may include the location of the phenomenon (e.g.,arrival direction, the extent and volume), a motion trajectory of thephenomenon (e.g., including its velocity, possible pathways), aclassification type or categorization of the phenomenon (e.g., based onweather type, severity level, and the like). The determinedcharacteristics (and the detection of the phenomenon itself) may beassociated with a probability metric representing a level of confidencevalue. Data analysis unit 120 may also determine possiblecountermeasures or techniques for avoiding or minimizing the adverseeffects arising from a detected phenomenon.

An operator of system 100 may establish or adjust the threshold levelsfor detection of an adverse airborne phenomenon, and may apply adaptivethresholds in real-time during the aircraft flight. The detectionthreshold levels may be selected manually, or established based ondefault or dynamically changing criteria. For example, a clear-airturbulence (CAT) phenomenon may have a different subjective impact onthe pilot as compared to the aircraft passengers. If the pilot considersthe current CAT detection thresholds to be too high, such as afterencountering a particular CAT event, then the pilot may reclassify CATsfor detection along the current aircraft flight route to include onlyCATs characterized with lower thresholds. In this manner, data analysisunit 120 will avoid unnecessary detections of CATs considered to be“light intensity”.

Data analysis unit 120 further generates an image of detected adverseairborne phenomena, which then overlaid onto a (wideband) image of theexternal environment which is displayed to the pilot (or other aircraftcrew member) via display unit 130, enabling the pilot to easily perceivethe extent of the airborne phenomenon with respect to the field of view.

In procedure 176, a visual representation characterizing the detectedadverse airborne phenomenon is dynamically displayed overlaid onto aview of the external environment displayed to an operator of theaircraft. Referring to FIG. 1, display unit 130 receives an imagerepresentation of adverse airborne phenomena detected by data analysisunit and presents the image representation to an operator of system 100.The displayed image may be a wideband video image, which is updated inreal-time in accordance with changes in the displayed airbornephenomena, and which is conformed to a view of the external scene.Display unit 130 may be at least partially transparent (i.e., a“see-through display”), such that the viewer can simultaneously observeimages or other visual content superimposed onto the display along witha view of the external environment through the display, providing theviewer with situational awareness. Display unit 130 may be embodied by ahead-mounted display (HMD) that includes a display embedded within awearable apparatus worn by the operator, such as a helmet, a headband, avisor, spectacles, goggles, and the like. Alternatively, display unit130 may be another type of display, such as: a head-up display (HUD),e.g., embedded in the aircraft cockpit; a portable or hand-held display;a display screen of a mobile computing device; and the like.

The wideband image of the detected phenomena superimposed on displayunit 130 may be based on the infrared images captured by imaging unit115, or by other cameras or detectors coupled with data acquisition unit110. The detected adverse airborne phenomena and their associatedcharacteristics may be displayed visually in various forms, such asimage objects, graphics, symbols, animations, and the like.

Reference is made to FIG. 11, which is a schematic illustration of adynamic display of detected adverse airborne phenomena, constructed andoperative in accordance with an embodiment of the present invention. Thevisual representation of the detected adverse airborne phenomenon issuperimposed onto a view of the external environmental within the fieldof view of the pilot. The displayed image includes an indication of thelocation of the detected phenomena, as well as additional informationsuch as a dynamic vector representing the motion trajectory of theairborne phenomenon, and the severity level. The phenomenoncharacteristics are presented synthetically using symbols, text, colors,audible cues, animation, magnification, and the like.

The visual representation of the detected phenomena may include variablevisual attributes representing different categories or classificationsof the adverse airborne phenomena and their characteristics. Forexample, a first detected phenomenon characterized with a “highseverity” may be visually depicted using a first variation of at leastone visual attribute, such as: a particular shape (e.g., a triangle), aparticular color (e.g., red), a particular contour (e.g., bold or highintensity outline), and/or a particular text notification (“warning—highseverity). Correspondingly, a second detected phenomenon characterizedwith a “medium severity” may be visually depicted using a secondvariation of the visual attribute(s), such as: a different shape (e.g.,a diamond), a different color (e.g., yellow), a different contour (e.g.,italics or medium intensity bolded outline), and/or a different textnotification (“warning—medium severity). Finally, a third detectedphenomenon characterized with a “low severity” may be visually depictedusing a third variation of the visual attribute(s), such as: anotherdifferent shape (e.g., a circle), another different color (e.g., green),another different contour (e.g., regular font or low intensity outline),and/or a particular text notification (“clear—low severity).

The presence of an adverse airborne phenomenon may be indicated using asolid or dashed contour line. A blinking contour may provide an alert ofa particular phenomenon, with the blinking frequency indicating theseverity level. Different visual symbols allow the viewer to distinguishone adverse airborne phenomenon form another, such that multiplephenomena can be displayed simultaneously. Physical characteristics ofthe adverse airborne phenomenon may be emphasized in different ways. Forexample, the concentration of the adverse airborne phenomenon may berepresented using colors, and a reference color bar may be presentedalongside the displayed image to assist interpretation. The velocity(speed and direction) of the adverse airborne phenomenon may be depictedusing arrows, where the arrow size represents the speed. Similarly, arelevant symbol bar may be presented on the displayed image to assistinterpretation of the various symbols. A further technique for visuallydepicting the severity level classification (e.g., direct impact on theaircraft and/or the passengers) of adverse airborne phenomena utilizesthe transparency level of the symbol, where for example, phenomenaassociated with higher severity levels will be depicted as opaque(lesser transparency) whereas those associated with lower severitylevels are depicted as transparent. Other visual attribute indicationsmay include: relative size (e.g., increasing or decreasing the size of asymbol); a color-related parameter or color attribute (e.g., changingthe brightness, hue, saturation, luminance, radiance of a symbol);emphasizing or highlighting a portion of the symbol; omitting a portionof the symbol; changing the type of symbol entirely; and the like.

The displayed visual representation facilitates the decision making ofthe pilot for avoiding or otherwise mitigating the effects of thedetected adverse airborne phenomenon along the flight route of theaircraft. Additional relevant information obtained from data acquisitionunit 110 and/or other aircraft systems may also be taken intoconsideration for determining an optical flight maneuver (e.g., toadjust the current flight route) or other suitable course of action.

Referring back to FIG. 10, the inter-image analysis performed by dataanalysis unit 120 may provide additional time-dependent informationrelating to the adverse airborne phenomenon. Such information may bedisplayed as predicted characteristics, such as time-dependentconcentration and position. For example, the position of the adverseairborne phenomenon while the aircraft passes nearby may be depictedusing animation showing the routes and positions of both the aircraftand the phenomenon according to its current velocity. Such a visualrepresentation may further assist the pilot in determining a suitablecourse of action in order to safely avoid the airborne phenomenon.

While certain embodiments of the disclosed subject matter have beendescribed, so as to enable one of skill in the art to practice thepresent invention, the preceding description is intended to be exemplaryonly. It should not be used to limit the scope of the disclosed subjectmatter, which should be determined by reference to the following claims.

The invention claimed is:
 1. An aircraft system for detecting andpresenting information relating to adverse airborne phenomena along aflight route of the aircraft, the system comprising: a data acquisitionunit, comprising an imaging unit configured to acquire a plurality ofinfrared (IR) images of the external environment of said aircraft, saidimaging unit comprising: an IR detector, configured to detect IRradiation; and a tunable spectral filter (TSF), configured toselectively transmit at least one IR wavelength band of incident lightto said IR detector, wherein said imaging unit is configured to acquirewideband IR images of said environment when operating in a first mode,and configured to acquire narrowband IR images of said environment, eachof said narrowband IR images respective of a different IR spectral band,when operating in a second mode; a data analysis unit, configured toreceive and process the IR images acquired by said imaging unit, todetect and determine at least one characteristic of at least one adverseairborne phenomenon in said environment, based on at least the spectralsignatures of environmental features in said narrowband IR images; and adisplay unit, configured to dynamically display a visual representationof the detected adverse airborne phenomenon and determinedcharacteristic thereof, overlaid onto a view of said externalenvironment displayed to an operator of said aircraft.
 2. The system ofclaim 1, wherein said data acquisition unit is configured to obtaininformation relating to said external environment from at least one datasource selected from the list consisting of: a weather/climate forecastmodel; a three-dimensional (3D) geographic model; a digital terrainelevation model (DTEM); a ground/satellite observation station; a birdmigration information source; another airborne platform; and anycombination of the above.
 3. The system of claim 1, wherein saidcharacteristic of the detected adverse airborne phenomenon is selectedfrom the list consisting of: a classification type of said detectedadverse airborne phenomenon; a location of said detected adverseairborne phenomenon; a motion trajectory of said detected adverseairborne phenomenon; a severity level of said detected adverse airbornephenomenon; and at least one technique for avoiding or mitigating theeffects of said detected adverse airborne phenomenon.
 4. The system ofclaim 1, wherein said data analysis unit is further configured tocompare said spectral signatures with predefined values or thresholds,to determine characteristics of said detected adverse airbornephenomenon.
 5. The system of claim 1, wherein said visual representationcomprises a plurality of variable visual attributes, each of said visualattributes representing a respective category of the characteristics ofsaid detected adverse airborne phenomenon.
 6. The system of claim 5,wherein said visual attributes is selected from the list consisting of:at least one symbol; at least one color; at least one color attribute;at least one contour; at least one shape; at least one alphanumericcharacter; a text notification; a highlighting indication; a flashingvisual attribute; a varying visual attribute; and any combination of theabove.
 7. The system of claim 1, wherein said display unit comprises atleast one display selected from the list consisting of: a head-updisplay (HUD); a head-mounted display (HMD); and a display embeddedwithin a wearable apparatus.
 8. The system of claim 1, wherein saidfirst mode comprises a scanning mode, in which said TSF is tuned totransmit all detectable IR wavelengths, and said IR detector isconfigured to acquire multispectral waveband images, wherein said dataanalysis unit is directed to detect all potential adverse airbornephenomena in said environment.
 9. The system of claim 1, wherein saidsecond mode comprises an investigation mode, in which said TSF is tunedto transmit selected IR wavelength bands, and said IR detector isconfigured to acquire narrowband images, wherein said data analysis unitis directed to determine further information relating to a potentialadverse airborne phenomenon detected when operating in said first mode.10. The system of claim 1, wherein said imaging unit is furtherconfigured to operate in a calibration mode, in which said TSF is tunedto block radiation from both directions, and said TSF is positioned soas to reflect radiation emitted from a black-body surface at preselectedtemperatures toward said IR detector, allowing for offset and gaincalibrations of said TSF.
 11. The system of claim 1, wherein said dataanalysis unit is configured to generate a contrast image from aplurality of narrowband images at different wavelength bands acquired bysaid imaging unit, and to determine a spectral signature from saidcontrast image.
 12. The system of claim 1, wherein said adverse airbornephenomenon is selected from the list consisting of: a selected quantityof atmospheric particles; a selected quantity of volcanic ash particles;a selected quantity of acid or halogen molecules; a selected quantity ofice particles; a selected quantity of water droplets; a selectedquantity of undercooled water droplets; a turbulence airflow; a verticalshear force; a convectively induced turbulence condition; a clear airturbulence (CAT) condition; the presence of birds; and any combinationof the above.
 13. A method for detecting and presenting informationrelating to adverse airborne phenomena along a flight route of anaircraft, the method comprising the procedures of: acquiring a pluralityof infrared (IR) images of the external environment of said aircraft,using an imaging unit comprising: an IR detector, configured to detectIR radiation; and a tunable spectral filter (TSF), configured toselectively transmit at least one IR wavelength band of incident lightto said IR detector, by acquiring wideband images of the environmentwhen operating in a first mode, and acquiring narrowband IR images ofthe environment, each of said narrowband IR images respective of adifferent IR spectral band, when operating in a second mode; processingwith a data analysis unit the IR images acquired by said imaging unit,to detect and determine at least one characteristic of at least oneadverse airborne phenomenon in said environment, based on at least thespectral signatures of environmental features in said narrowband IRimages; and dynamically displaying a visual representation of thedetected adverse airborne phenomenon and determined characteristicsthereof, overlaid onto a view of said external environment displayed toan operator of said aircraft.
 14. The method of claim 13, wherein saidcharacteristic of the detected adverse airborne phenomenon is selectedfrom the list consisting of: a classification type of said detectedadverse airborne phenomenon; a location of said detected adverseairborne phenomenon; a motion trajectory of said detected adverseairborne phenomenon; a severity level of said detected adverse airbornephenomenon; and at least one technique for avoiding or mitigating theeffects of said detected adverse airborne phenomenon.
 15. The method ofclaim 13, wherein said visual representation comprises a plurality ofvariable visual attributes, each of said visual attributes representinga respective category of the characteristics of said detected adverseairborne phenomenon.
 16. The system of claim 15, wherein said visualattributes is selected from the list consisting of: at least one symbol;at least one color; at least one color attribute; at least one contour;at least one shape; at least one alphanumeric character; a textnotification; a highlighting indication; a flashing visual attribute; avarying visual attribute; and any combination of the above.
 17. Themethod of claim 13, wherein said first mode comprises a scanning mode,in which said TSF is tuned to transmit all detectable IR wavelengths,and said IR detector is configured to acquire multispectral wavebandimages, wherein said data analysis unit is directed to detect allpotential adverse airborne phenomena in said environment.